Plastic Crack Opening Displacement Research Articles

Fully plastic crack opening analyses of complex-cracked pipes for Ramberg-Osgood materials

The plastic influence functions for calculating fully plastic Crack opening displacement (COD) of complex-cracked pipes were newly proposed based on systematic 3-dimentional (3-D) elastic-plastic Finite element (FE) analyses using Ramberg-Osgood (R-O) relation, where global bending moment, axial tension and internal pressure are considered separately as a loading condition. Then, crack opening analyses were performed based on GE/EPRI concept by using the new plastic influence functions for complex-cracked pipes made of SA376 TP304 stainless steel, and the predicted CODs were compared with FE results based on deformation plasticity theory of tensile material behavior. From the comparison, the confidence of the proposed fully plastic crack opening solutions for complex-cracked pipes was gained. Therefore, the proposed engineering scheme for COD estimation using the new plastic influence functions can be utilized to estimate leak rate of a complex-cracked pipe for R-O material.

Plastic Limit Loads for Slanted Through-Wall Cracks in Cylinder and Plate Based on Finite Element Limit Analyses

The plastic limit load solutions for cylinder and plate with slanted through-wall cracks (TWCs) are developed based on the systematic three-dimensional (3D) finite element (FE) limit analyses. As for loading conditions, axial tension, global bending, and internal pressure are considered for a cylinder with slanted circumferential TWC, whereas, axial tension and internal pressure are considered for a plate and a cylinder with slanted axial TWC. Then, the verification of FE model and analysis procedure employed in the present numerical work was confirmed by employing the existing solutions for both cylinder and plate with idealized TWC. Also, the geometric variables of slanted TWC which can affect plastic limit loads were considered. Based on the systematic FE limit analysis results, the slant correction factors which represent the effect of slanted TWC on plastic limit load were provided as tabulated solutions. By adopting these slant correction factors, the plastic limit loads of slanted TWC can be directly estimated from existing solutions for idealized TWC. Furthermore, the modified engineering estimations of plastic limit loads for slanted TWC are proposed based on equilibrium equation and von Mises yield criterion. The present results can be applied either to diverse structural integrity assessments or for accurate estimation of fracture mechanics parameters such as J-integral, plastic crack opening displacement (COD) and C*-integral for slanted TWC based on the reference stress concept (Kim, et al., 2002, “Plastic Limit Pressure for Cracked Pipes Using Finite Element Limit Analyse,” Int. J. Pressure Vessels Piping, 79, pp. 321–330; Kim, et al., 2001, “Enhanced Reference Stress-Based J and Crack Opening Displacement Estimation Method for Leak-Before-Break Analysis and Comparison With GE/EPRI Method,” Fatigue Fract. Eng. Mater. Struct., 24, pp. 243–254; Kim, et al., 2002, “Non-Linear Fracture Mechanics Analyses of Part Circumferential Surface Cracked Pipes,” Int. J. Fract., 116, pp. 347–375.)

Crack opening displacement and propagation rate of microstructurally short cracks

Crack opening displacement (COD) along the edges of microstructurally short fatigue cracks in austenitic stainless steel was investigated in situ in the SEM. The measured plastic part of the COD range between maximum and minimum load was extrapolated to the near crack tip region, assuming plastic blunting and resharpening by cooperative slip as well known for long cracks. In accordance with the blunting model, the measured crack propagation per cycle is about the half of the extrapolated plastic COD range.

Study of the fracture behavior in soft and hard oriented NiAl single crystals by AFM

In order to understand the fracture behavior and room temperature brittleness of NiAl single crystals, atomic force microscopy (AFM) investigations were performed on in-situ loaded 4-point-bend specimens of hard and soft oriented crystals. A significant amount of dislocations are emitted from the crack tip on different slip systems. {1 0 0} slip planes are activated in soft crystals, whereas in hard crystals also slip planes, which are identified as {1 1 2} or {1 2 3} are observed. From evaluations of the AFM images, dislocation distributions ahead of crack tips and the plastic crack opening displacement (COD) from blunted crack tips are examined. The fracture toughness is calculated from the measured plastic COD and compared with the results obtained from load-displacement curves. First in-situ experiments on NiAl specimens heated up to 400 K are described, where a significant increase in plasticity is observed.

Dynamic cracking resistance of ChSN 15313 steel

1. The method of stereofractographic examination of the stretched zone on two fracture halves was used to determine the values of 2h for ChSN 15313 steel in the temperature range 73–253°K in dynamic loading. 2. A correlation was found in the given temperature range between the height of the stretched zone and crack opening displacement predicted by the mathematical models. The coefficient of linear correlation between 2h and K2Qd/Eσ0.2d is equal to 0.9. 3. The stretched zone does not form at temperatures below 153°K. 4. The correlation between JId and δc in dynamic loading was verified by means of experiments. The derived equation makes it possible to determine the dynamic fracture toughness KJd on the basis of the values of KQd, the dynamic yield stress, and the height of the stretched zone 2h, characterizing the plastic crack opening displacement, in the conditions of elastoplastic loading. 5. The results indicate that the quantitative stereofractographic method can be used successfully for evaluating the fracture resistance of the materials, especially in the impact loading conditions in which the determination of crack opening displacement by other methods is difficult.